During the last several decades, ordered intermetallics (based on aluminides of transition metals such as nickel, iron, titanium, niobium, and cobalt) have been under investigation for their possible use as high-temperature structural materials. Aluminides of transition metals possess sufficiently high concentrations of aluminum to form a continuous, fully adherent alumina layer on the surface when exposed to air or oxygen atmospheres. The amount of aluminum present in aluminides can range from 10 to 30 wt % and is significantly higher than the aluminum concentrations present in conventional alloys and superalloys. In the case of nickel and iron aluminides, the alumina layer formed on the surface of the materials is responsible for their excellent oxidation and carburization resistances even at temperatures of 1000.degree. C. or higher. Therefore, aluminides, unlike conventional steels and superalloys based on nickel, iron, and cobalt, do not necessarily require chromium to form an oxide layer on the surface of the material to protect against high-temperature oxidation and corrosion. Alumina is much more thermodynamically stable at high temperatures than Cr.sub.2 O.sub.3.
Aluminides possess lower densities and high melting points, and exhibit desirable mechanical properties due to their ordered crystal structures. The strength of some intermetallics increases with temperature instead of decreasing; thus, they are ideally suited for high-temperature applications. Advantages of intermetallics based on Ni.sub.3 Al, Fe.sub.3 Al, and FeAl include:
(1) resistance to oxidation and carburization in both oxidizing and reducing carburizing atmospheres up to 1100.degree. C.; PA1 (2) better tensile and compressive yield strengths at 650 to 1100.degree. C. than many nickel-based superalloys; PA1 (3) fatigue resistance superior to that of nickel-based superalloys, PA1 (4) superior creep strength; PA1 (5) excellent wear resistance at temperatures above 600.degree. C. (wear resistance increases with temperature, similarly to yield strength, and the increase can be as much as a factor of 1000); and PA1 (6) the formation of alumina on the surface by preoxidation, which provides good chemical compatibility for many environments.
Very few materials provide fully adherent, robust and stable alumina surface films even at 1000.degree. C. Most of the existing ferrous and nonferrous alloys contain less than 2 wt % Al, and can only provide a chromia scale. Some of the superalloys contain about 5 wt % Al, and the alumina film formed on their surfaces is not fully protective. Superalloys are generally coated with NiAl by chemical vapor deposition such that the surface provides alumina film on exposure to oxygen at high temperatures. In some cases, superalloys have been coated with platinum to provide a platinum aluminide coating on the surface. Platinum aluminide coatings are known to be effective in providing an adherent alumina layer on exposure to oxygen.
While alumina layers can be obtained on superalloys, the presence of high chromium content and low aluminum content is known to permit degradation in superalloys over a period of time. Also, chemical vapor deposition (CVD) of aluminide coatings is an expensive and time-consuming process, and to date only the aircraft industry makes significant use of these coatings for refurbishing turbine engine components. Chromia formed on the surfaces of most alloys is not thermodynamically stable beyond 900.degree. C. and tends to peel off when thermally cycled. While chromia is considered a reasonable insulator at room temperature, its electrical resistivity falls at high temperatures rendering it unsuitable for use as a substrate for electronic circuits. Differences in coefficients of thermal expansion between the chromia and conductive elements applied by today's thick-film pastes make it unlikely that any commercial alloy with a chromia surface layer can be used as a substrate for electronic circuits. Since commercial thick-film pastes require firing temperatures of 950.degree. C. and are thus only compatible with alumina, additional materials, manufacturing process, and techniques must be developed before they can be used with other substrate materials.
Today's printed electronic circuits are built on substrates made of polymers, plastics, ceramics, and other materials. Alumina substrates are used in both thick film and thin film embodiments. Examples include 96% alumina sold by Kyocera Corporation, S-22 Kiainoue-cho, Higashino, Yamashina-Ku, Kyoto 67, Japan and thick film alumina tape with borosilicate binder sold by DuPont Corporation, Wilmington, Del. Both are fragile, and both have relatively low thermal conductivities. Higher thermal conductivities are desirable for use in state-of-the-art power integrated circuit chips, in which it is desirable to dissipate heat in the range of 50 to 100 Watts per square centimeter (W/cm.sup.2). Efforts have been and are currently being made world-wide to find or develop materials with the desired high-temperature strength and high thermal conductivities for these applications.
It is therefore desirable to provide substrates which can utilize existing materials, technologies, and equipment while also having more desirable mechanical integrity and thermal dissipation characteristics.